Good vibrations: plants respond to sound of caterpillars chewing

Plants are smarter than you think. Not only do they grow towards sunlight and respond to rock music, but they can also learn from the sound of their predators, according to new research.

Two University of Missouri scientists recorded the miniscule vibrations made by caterpillars feasting on thale cress leaves. Using special technology, they replayed the sound to the plants, and discovered those which were "primed" with the recording were better able to fight back against a caterpillar attack than those which remained in silence.

The treated plants produced higher levels of key chemical defences, the study found.

Sydney University biologist Robyn Overall said she was not surprised by the results.

"Plants are very clever. People always seem surprised by that," she said.

"Unlike animals, they have a lot of mechanisms where they’re finely tuned to respond to their environment."

Plants exposed to the recorded vibrations produced more glucosinolates (mustard oils) and anthocyanins, which give colour to flowers and red wine, and provide health benefits in dark chocolate.

In a second experiment, researchers exposed the plants to similar environmental vibrations made by light winds and the mating song of a common treehopper. But neither of those elicited the same chemical defence as the sound of a caterpillar's chomping.

"Plants thus respond to herbivore-generated vibrations in a selective and ecologically meaningful way," concluded the study, which was published in the peer-reviewed ecology journal Oecologia.

Professor Overall said the research demonstrated an artificial version of the learning process which plants naturally undertake – more experienced plants resist attack better than less experienced plants. But she said the study could have implications for the agricultural industry, perhaps reducing the need for artificial pesticides to keep bugs away from crops.

Plants have complex defence mechanisms, both chemical and electrical. Some can even emit "volatile compounds" – small molecules released into the air to repel insects or attract natural predators of those insects.

They can also communicate with each other. The study noted that under the right conditions, "vibrations generated by a chewing caterpillar could alert nearby plants to the presence of a herbivore".

For Professor Overall, it is a testament to the work she has performed her entire career.